Photonic Integrated Devices (PIDs) that include Photonic Integrated Circuits (PICs) are monolithically integrated to achieve different optical functionalities. These PICs enable production of complex optical circuits using high volume semiconductor wafer fabrication techniques. Further, the PICs offer to reduce component footprint and eliminate multiple packaging issues and multiple optical alignments. These PICs find application in optical communication networks and mass production of consumer photonics products.
In the context of applications, the advantages of PICs become especially compelling when active waveguide devices, such as laser or photodetector, are combined with one or more passive waveguide devices and elements of the waveguide circuitry, to form a highly functional PIC on a chip with minimal ports. The active waveguide devices that modulate optical signals by electrical means are usually made from artificially grown semiconductors having bandgap structures adjusted to the function and wavelength range of their particular application. Such semiconductors are a natural choice for the base material of the PICs. Accordingly, semiconductor based PICs in which several functions such as optical signal detection, optical modulation, and optical signal emission are implemented in a single monolithic semiconductor chip are a promising solution. Further, indium phosphide (InP) and its related III-V semiconductor material system offer additional benefits as they allow the fabrication of active devices operating in the important wavelength ranges around 1300 nm and 1550 nm, i.e., in the two dominant low-loss transmission windows of the glass fibers. However, even such monolithic integration can provide cost barriers with poor design methodologies, low manufacturing yields, complicated manufacturing processes, and repeated expensive epitaxial growth processes. Accordingly, single step epitaxial wafer growth methodologies in conjunction with established wafer fabrication technologies, have received attention as a means to further enable reduced optical components cost.
Alternatively, gallium arsenide (GaAs) and aluminum gallium arsenide (AlGaAs) may be employed for 850 nm and 1300 nm PICs. Further, PICs may be employed across visible and near ultraviolet ranges through exploitation of other tertiary and ternary semiconductor materials employing indium (In), gallium (Ga), aluminum (Al), arsenic (As), and phosphorous (P). The function of any waveguide device within a PIC composed of epitaxially grown semiconductor heterostructures is pre-determined by its band structure and, more particularly by the bandgap wavelength of the waveguide core layer, cladding layer, and substrate. Accordingly, functionally different devices are typically made from different, yet compatible, semiconductor materials although, through targeted design, some structures can provide for example, optical amplification and photodetection with reversed bias polarity. However, the selection of the substrate and waveguide design has a profound impact both on the design and fabrication of the PIC.
In several PICs ranging from wavelength division multiplexers (WDMs), wavelength division demultiplexers (also referred to as WDMs), optical power (channel) monitors, reconfigurable optical add-drop multiplexers (ROADMs), and dynamic gain (channel) equalizers (DGEs/DCEs), at least one multi-wavelength signal is spectrally dispersed, detected, monitored, and processed on a per wavelength basis. For an array of multi-wavelength signals, the array of multi-wavelength signals are monitored and processed on a per wavelength basis, and then multiplexed to form a multi-wavelength outgoing signal. Such PICs must operate on predetermined channel wavelength plans (i.e., O-band (Original; 1260 nm≤λ≤1360 nm); E-band (Extended; 1360 nm≤λ≤1460 nm); S-band (Short; 1430 nm≤λ≤1530 nm); C-band (Conventional; 1530 nm≤λ≤1565 nm); and L-band (Long; 1565 nm≤λ≤1625 nm) as the optical signals having specific wavelengths are generally provided from a plurality of remote and discrete transmitters. The channel wavelength plans are defined by the International Telecom Union in ITU-T G.694.1 “Spectral Grids for WDM applications: DWDM Frequency Grid.” Accordingly, this defines a fixed grid exploiting channel spacings of 12.5 GHz, 25 GHz, 50 GHz, and 100 GHz according to the equation (1) as shown below:193.1THz+n*Spacing/1000  (1)where Spacing=12.5 GHz, 25 GHz, 50 GHz, and 100 GHz, and n≥0 within the C and L bands of the optical spectrum.
There is also a flexible grid with channels centered at 193.1 THz+n×0.00625 where n≥0, i.e., 6.25 GHz centers, and channel bandwidths defined by 12.5 GHz*m, where m≥0. Instead of dense WDM (DWDM) other systems exploit coarse WDM (CWDM) as specified by ITU-T G.694.2 defining wavelengths from 1271 nm through 1611 nm with a channel spacing of 20 nm.
The temperature stability of the PICs that include optical emitters such as laser diodes (LDs) become a design constraint over operating temperature range 0° C.≤T≤70° C., i.e., during internal customer premises and telecom installations, and −40° C.≤T≤85° C. for external plant. Further, even low temperature dependencies in terms of nm/° C. become significant at channel spacings of ˜0.8 nm (100 GHz) and ˜0.4 nm (50 GHz). In an example, InP exhibits a temperature sensitivity of ˜0.1 nm/° C. such that over 0° C.≤T≤70° C. the wavelength will shift ˜6 nm or ˜8/˜15 channels at 100 GHz/50 GHz respectively. As such, temperature control through heaters and thermoelectric coolers have become a standard for today's deployed discrete LDs in most DWDM and CWDM applications except where low channel counts with wide channel spacing and significant guardbands are specified to enable uncooled LDs and PICs where superluminescent light emitting diodes (SLEDs) are employed.
As we move from considering a discrete distributed feedback (DFB) LD to a 4-channel, a 16-channel, and a 40-channel PIC with integrated CWDM/DWDM MUX, the die footprint increases significantly, such that active temperature stabilization becomes increasingly difficult to achieve. Further, there are additional issues that arise with integration such as thermal crosstalk between adjacent elements and the like. Referring now to FIG. 1, temperature dependent wavelength offsets of InP and SiO2 echelle gratings according to designs of the prior art are shown. A First image 100A shows an expected transmission shift of one channel of an InP Echelle grating WDM with a Gaussian passband characteristic. The peak shifts approximately by +7.6 nm over 85° C. corresponding to dλ/dT≈+0.09 nm/° C. Accordingly, in order to deploy such an InP WDM the effective dn/dTAMB of the WDM must be modified by some form of compensation so that the effect of ambient temperature, TAMB, is reduced. Within the prior art this may be through exploiting a thermoelectric cooler to maintain the InP die temperature at a nominal value, i.e., TInP=35° C. or through the employment of on-chip micro-heaters exploiting resistive metal traces such that the nominal InP die temperature is set above the maximum operating temperature, i.e. TAMB=70° C.-85° C. in order to avoid control issues at TInP=100° C. Within the prior art it is also known that compensating for the inherent refractive index change of a material can be compensated by integrating a second waveguide section with the opposite dn/dT or by modifying the waveguide design to include a cladding material with a negative dn/dT such that the index change of the waveguide due to change in temperature is reduced. However, heaters and thermo-electric coolers can require significant electrical power consumption and also impose complex thermal management requirements upon the die packaging even for a passive DWDM to ensure uniform temperature before active devices are considered. Further, negative temperature coefficient materials, i.e., dn/dT<0, are typically polymeric and have low coefficients such that compensating a high dn/dT material such as InP requires significant waveguide real estate to achieve the desired balance. However, other waveguide material systems provide different dn/dT and hence dλ/dT. Referring now to second image 100B, the expected transmission shift of one channel of an SiO2 Echelle grating WDM with a Gaussian passband characteristic is shown. Compared to dnInP/dT≈2×10−4 silica offers dnSiO2/dT 2×10−5 such that over 85° C. the center wavelength shifts ≈0.8 nm which is equivalent to dλ/dT≈+0.009 nm/° C., an order of magnitude lower than InP. Thus, when such InP based PIC is configured as an optical emitter, the optical emitter will not generate an optical signal at its predetermined wavelength.
Further, certain applications such as optical line termination (OLT) or optical network unit (ONU) outside a plant portion, residence, and business within passive optical networks, include an optical source on each server line card within a server rack of a data center. These optical sources utilize a single emitter such as a DFB laser. The single emitter includes an active layer that is periodically structured as a diffraction grating layer to provide an optical feedback for the single emitter. However, manufacturing of such optical emitter with the diffraction grating layer can provide cost barriers with poor design methodologies, low manufacturing yields, and complicated manufacturing processes. Further, in conventional DFB lasers, the shift in wavelength of an optical signal generated by the DFB lasers is dependent on temperature. Thus, such DFB lasers do not provide a single mode of operation.
A known technique to overcome problem of the temperature dependence is to fabricate a PIC formed on a silicon substrate that includes a dielectric slab as described in Koteles et al in US Patent entitled “Athermal Waveguide Grating based Device having a Temperature Compensator in the Slab Waveguide Region”. The dielectric slab has a predetermined thickness, and is spaced apart from the grating element. However, when such PIC is formed using silicon substrate, there is mismatch between the lattice structure of the InP structure and the silicon substrate. Thus, the InP structure cracks due to high stress between the InP structure and the silicon substrate.
Another known technique to overcome the problem is to alter the structure of the optical emitter. Referring now to FIG. 2, an edge-emitting laser 200A is shown. The edge-emitting laser 200A includes a pair of dielectric interference filters formed on opposing facets.
A graph 200B shows an ideal wavelength characteristic of the edge-emitting laser 200A. Each dielectric interference filter is a Fabry-Perot etalon (FP-E) filter. The edge-emitting laser 200A has a resonant dielectric cavity that is sandwiched between the pair of dielectric interference filters. The dielectric interference filter manifests a flat reflectance spectrum with a deep reflectance notch at the center. Accordingly, each FP-E filter transmits over a narrow wavelength range such that the round trip loss of the cavity is high when the wavelength overlaps either of the FP-E filter transmissive ranges. Hence, the characteristic response shows a double-peaked shape with a sharp minimum between the two notch wavelengths, where the sharp minimum determines a lasing wavelength of the optical emitter. However, such cavity losses in the edge-emitting laser 200A lead to a high threshold for lasing, low output power, gain saturation, and wavelength crosstalk.
The semiconductor die acts as a Fabry-Perot cavity when the refractive index of the semiconductor die is greater than the refractive indices of the FP-E filters. Thus, the resultant wavelength characteristic of the edge-emitting laser 200A is actually closer to that depicted in graph 200C where there is no sharp peak in the reflectance. Such edge-emitting laser 200A functions like a broadband superluminescent LED.
It would therefore be beneficial to provide PIC designers with design methodologies for the single and multi-wavelength LDs that are not only compatible with monolithic integration on compound semiconductor PICs but also provide for athermal performance, resolving thermal management issues, and consumption of less power.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.